Me parece raro que la biblioteca esté abierta tan tarde.

Me parece means it seems to me or I find it. The me shows who has the impression.

So:

• Me parece raro... = It seems strange to me... / I find it strange...
• Te parece raro... = It seems strange to you...
• Le parece raro... = It seems strange to him/her...

This structure is very common in Spanish for giving opinions in a slightly softer, less direct way than a plain statement.

Because the subject of parece is not I. In me parece raro que..., the idea is literally something like:

• It seems strange to me that...

So parece is the él/ella/usted form, but here it is really being used impersonally: it seems.

Compare:

• Yo parezco cansado = I look/seem tired
• Me parece raro = It seems strange to me

These are different uses of parecer.

Que introduces the clause that explains what seems strange.

• Me parece raro que la biblioteca esté abierta tan tarde.

It connects the main idea (It seems strange to me) with the thing being judged (that the library is open so late).

This is very common after expressions like:

• Es bueno que...
• Es raro que...
• Me molesta que...
• Me alegra que...

Because this sentence uses the subjunctive after an expression of opinion or evaluation about a situation: me parece raro que...

So:

• Me parece raro que la biblioteca esté abierta...

The main clause expresses a reaction or judgment, not a simple fact. In Spanish, that often triggers the subjunctive in the que clause.

A native English speaker often wants to use the indicative here, but Spanish normally prefers the subjunctive after phrases like:

• Es raro que...
• Es normal que...
• Me sorprende que...
• No creo que...

Esté is the present subjunctive of estar.

The forms are:

• yo esté
• tú estés
• él/ella/usted esté
• nosotros estemos
• vosotros estéis
• ellos/ellas/ustedes estén

Here we use esté because the subject is la biblioteca, which is singular.

Because estar abierta describes a state or condition: the library is currently open.

• La biblioteca está abierta = The library is open

With buildings, shops, offices, libraries, etc., Spanish normally uses estar abierto/a for whether they are open or closed at a particular time.

Compare:

• La puerta está abierta = The door is open
• La tienda está cerrada = The shop is closed

Using ser abierta would usually not mean the same thing.

Because abierta agrees with la biblioteca, which is feminine singular.

• la biblioteca → feminine singular
• so: abierta

Examples:

• El museo está abierto
• La biblioteca está abierta
• Los museos están abiertos
• Las bibliotecas están abiertas

This is basic adjective agreement.

Tan tarde means so late.

In this sentence, tan is used because the speaker is reacting to the degree of lateness:

• tan tarde = so late

Muy tarde means very late, which is possible in other contexts, but here tan tarde sounds more natural with the idea of I find it strange that...

Compare:

• La biblioteca está abierta muy tarde. = The library is open very late.
• Me parece raro que la biblioteca esté abierta tan tarde. = I find it strange that the library is open so late.

In Spanish, nouns usually need an article more often than in English.

So Spanish says:

• la biblioteca = the library

Even when English might sometimes sound natural without focusing on the article, Spanish generally keeps it.

Also, biblioteca is a feminine noun, so it takes la.

Yes. That is completely natural.

• Es raro que la biblioteca esté abierta tan tarde. = It’s strange that the library is open so late.
• Me parece raro que la biblioteca esté abierta tan tarde. = It seems strange to me that the library is open so late.

The version with me parece makes the reaction a bit more personal: it highlights that this is your impression.

No. In this sentence, raro means strange, odd, or unusual, not rare.

This is a very common false friend for English speakers.

Examples:

• Qué raro. = How strange.
• Es raro que no haya venido. = It’s strange that he/she hasn’t come.

If you want rare in the sense of uncommon, Spanish often uses poco común, inusual, or sometimes raro depending on context, but you should not automatically translate English rare as raro every time.

The structure is:

• Me parece raro
  • que
    • subjunctive clause

So:

1. Me parece raro = I find it strange / It seems strange to me
2. que la biblioteca esté abierta tan tarde = that the library is open so late

This pattern is very useful and productive. You can change the adjective:

• Me parece bien que... = It seems good to me that...
• Me parece mal que... = I think it’s wrong that...
• Me parece increíble que... = I find it unbelievable that...

And the verb after que will often go in the subjunctive.

Yes, it sounds completely natural in Spain.

It is standard, neutral Spanish and would be understood anywhere in the Spanish-speaking world as well. Nothing in the sentence is especially regional. The only specifically Spain-related point is that learners of Spain Spanish may also be used to seeing vosotros forms in other sentences, but this one does not involve that.